Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Also, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes one or more stimulation leads implanted at the desired stimulation site and an implantable neurostimulator, such as an implantable pulse generator (IPG), implanted remotely from the stimulation site, but coupled either directly to the stimulation leads or indirectly to the stimulation leads via one or more lead extensions in cases where the length of the stimulation leads is insufficient to reach the IPG. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation leads to stimulate the tissue and provide the desired efficacious therapy to the patient.
In the context of an SCS procedure, one or more stimulation leads are introduced through the patient's back into the epidural space, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. One type of commercially available stimulation leads is a percutaneous lead, which comprises a cylindrical body with ring electrodes, and can be introduced into contact with the affected spinal tissue through a Touhy-like needle, which passes through the skin, between the desired vertebrae, and into the epidural space above the dura layer. For unilateral pain, a percutaneous lead is placed on the corresponding lateral side of the spinal cord. For bilateral pain, a percutaneous lead is placed down the midline of the spinal cord, or two or more percutaneous leads are placed down the respective sides of the midline of the spinal cord, and if a third lead is used, down the midline of the special cord. After proper placement of the stimulation leads at the target area of the spinal cord, the leads are anchored in place at an exit site to prevent movement of the stimulation leads. To facilitate the location of the neurostimulator away from the exit point of the stimulation leads, lead extensions are sometimes used.
Whether lead extensions are used or not, the proximal ends of the stimulation leads exiting the spinal column are passed through a tunnel subcutaneously formed along the torso of the patient to a subcutaneous pocket (typically made in the patient's abdominal or buttock area) where a neurostimulator is implanted. The subcutaneous tunnel can be formed using a tunneling tool over which a tunneling straw may be threaded. The tunneling tool can be removed, the stimulation leads threaded through the tunneling straw, and then the tunneling straw removed from the tunnel while maintaining the stimulation leads in place within the tunnel.
The stimulation leads are then connected directly to the neurostimulator by inserting the proximal ends of the stimulation leads within one or more connector ports of the IPG or connected to lead extensions, which are then inserted into the connector ports of the IPG. The IPG can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted tissue, and in particular, the dorsal column and dorsal root fibers within the spinal cord.
The stimulation creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. Intra-operatively (i.e., during the surgical procedure), the neurostimulator may be operated to test the effect of stimulation and adjust the parameters of the stimulation for optimal pain relief. The patient may provide verbal feedback regarding the presence of paresthesia over the pain area, and based on this feedback, the lead positions may be adjusted and re-anchored if necessary. A computerized programming system, such as Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation, can be used to facilitate selection of the stimulation parameters. Any incisions are then closed to fully implant the system. Post-operatively (i.e., after the surgical procedure has been completed), a clinician can adjust the stimulation parameters using the computerized programming system to re-optimize the therapy.
The efficacy of SCS is related to the ability to stimulate the spinal cord tissue corresponding to evoked paresthesia in the region of the body where the patient experiences pain. Thus, the working clinical paradigm is that achievement of an effective result from SCS depends on the neurostimulation lead or leads being placed in a location (both longitudinal and lateral) relative to the spinal tissue such that the electrical stimulation will induce paresthesia located in approximately the same place in the patient's body as the pain (i.e., the target of treatment). If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy.
Multi-lead configurations have been increasingly used in electrical stimulation applications (e.g., neurostimulation, cardiac resynchronization therapy, etc.). In the neurostimulation application of SCS, the use of multiple leads increases the stimulation area and penetration depth (therefore coverage), as well as enables more combinations of anodic and cathodic electrodes for stimulation, such as transverse multipolar (bipolar, tripolar, or quadra-polar) stimulation, in addition to any longitudinal single lead configuration.
Several studies have demonstrated the advantage of using narrowly spaced, parallel leads placed symmetrically on both sides of the physiological midline in improving penetration and paresthesia coverage (see J. J. Struijk and J. Holsheimer Tripolar Spinal Cord Stimulation: Theoretical Performance of a Dual Channel System, Medical and Biological Engineering and Computing, Vol. 34, No. 4, 1996, pp. 273-279; J. Holsheimer, B. Nuttin, G. King, W. Wesselink, J. Gybels, and P. de Sutter, Clinical Evaluation of Paresthesia Steering with a New System for Spinal Cord Stimulation, Neurosurgery, Vol. 42, No. 3, 1998, pp. 541-549; Holsheimer J., Wesselink, W. A., Optimum Electrode Geometry for Spinal Cord Stimulation: the Narrow Bipole and Tripole, Medical and Biological Engineering and Computing, Vol. 35, 1997, pp. 493-497).
For example, to produce the feeling of paresthesia without inducing discomfort or involuntary motor movements within the patient, it is often desirable to preferentially stimulate nerve fibers in the dorsal column (DC) nerve fibers, which primarily include sensory nerve fibers, over nerve fibers in the dorsal root (DR) nerve fibers, which include both sensory nerve fibers and motor reflex nerve fibers. In order to stimulate the DC nerve fibers, while guarding against the stimulation of the DR nerve fibers, a transverse tripolar lead arrangement may activate anodes that flank a single cathode in a medial-lateral electrical field, with the single cathode providing the stimulation energy for the DC fibers, while the flanking anodes guarding against the over-stimulation of the DR fibers, thereby increasing the therapeutic range of SCS for stimulating the desired DC fibers, while reducing the unwanted side effect of stimulating DR fibers
Thus, in a multi-lead system, more particularly a system using multiple percutaneous leads, it is desired that two or more leads are placed parallel in close proximity to each other. During the lead implantation, the leads are placed closely at the surgeon's best effort. Fluoroscopy images are usually acquired after lead insertion to verify the placement and proximity of the leads, and a revision/correction can be made if necessary. Since the leads are ultimately placed in three-dimensional space, two-dimensional fluoroscopic views (e.g., in the context of SCS, an anteroposterior (AP) view and a lateral view) are used to check the lead proximity.
However, fluoroscopic imaging requires a bulky instrument, which limits its use in the operating room. Thus, sometimes, only an AP view is acquired, while a lateral view is optional and its acquisition depends on the preference of the surgeon. When only an AP view is acquired, it is usually assumed that there is no offset of the leads on the lateral view. However, in some cases, the visual estimate of lead proximity from an AP fluoroscopic image may incorrectly indicate that the leads are in close proximity, when in fact, the leads may be actually be quite separated from each other; something that may only be detected from a lateral fluoroscopic image of the leads. If such lead placement is not detected in a timely manner before the system is fully implanted, it may result in inefficient therapy and possibly require a second surgery for lead revision.
In addition, when programming a transverse tripolar system, knowing which electrode is in the middle of the medio-lateral electrode arrangement is absolutely critical to selecting the cathode that provides the stimulation, as well as selecting the anodes that provide the guarding function. Once the leads have been implanted, identifying the middle stimulation lead can be challenging.
Oftentimes, multiple leads may extend from the spinal region of the patient. For example, multiple percutaneous leads may be implanted within the patient adjacent the spinal cord. Because the programming of the IPG will depend upon the physical locations of the electrodes relative to the patient's spinal cord (especially in the case of a tripolar system as just described), the proximal ends of the leads may be labeled before passing them through the tunneling straw, so that the surgeon can keep track of which set of electrodes is connected to which connector port on the implanted IPG (which may include three ports for a medio-lateral electrode arrangement).
One technique used by surgeons to identify the leads is to tie sutures around the proximal ends of the leads prior to introducing them through the tunneling straw; for example, one suture around a first lead, two sutures around a second lead, three sutures around a third lead, etc. Once the proximal ends of the leads exit the tunneling straw, the surgeon can then identify each lead (and thus the corresponding electrodes) by the number of sutures tied to the respective lead, thereby allowing the lead to be connected to the correct port on the IPG.
While this technique can be successfully employed to identify leads, it considerably extends the length of the surgery time, which is undesirable. In some cases, the identification features, such as different colors or markings, can be incorporated into the proximal ends of the leads, such that the leads can be identified as they exit the tunneling straw. Even with the use of visual identifiers, however, the proximal ends of the leads can still be inserted into the incorrect connector ports, or the distal ends of the leads may have been mixed up during lead insertion, and therefore, the visual identifiers will not correspond to their intended electrodes. If the leads are inserted into the incorrect connector ports, intra-operative testing of the lead placement may be compromised. Additional surgical time may be wasted to identify and correct the connection problem. If the errors remain unidentified, the patient may leave the operating room with the leads incorrectly connected. During post-operative fitting, additional time may then be lost identifying and compensating for leads that are not in the proper connector ports. This ultimately can result in sub-optimal therapy.
Another technique that can be used to identify the leads is to individually activate stimulation for each lead and request the patient to provide paresthesia feedback (e.g., feeling from left, right, or both sides of the body) in order to determine the medio-lateral order of the leads. This could be time-consuming and may become confusing if the middle lead is placed laterally to the spinal cord physiological midline. Also, this conventional method may not be able to reveal the relative proximity of the two lateral leads absent a fluoroscopic procedure.
Additionally, in the case where multiple percutaneous leads are used to construct the medio-lateral electrode arrangement, knowing the relative proximity of the lateral leads to the middle lead is also helpful in sculpting the current/voltage applied to each guarding anode. Furthermore, the leads may not be oriented perfectly parallel, but rather tilted at an angle. In such cases, knowing the proximity (and in particular, the separation between two adjacent cross-lead electrodes) and relative orientation of the leads to each other may be critical to sculpting the stimulation current/voltage applied to each active electrode.
There, thus, remains a need for a quick, effective, and low-cost method for determining the relative proximity and orientation between two or more neurostimulation leads and/or identifying the middle lead of a tri-lead system.